Using a methodology that combines two new techniques, ultra-low temperature microdissection (ULTM) and the intracellular reference phase (RP), we have been able to unambiguously demonstrate sodium, potassium and water binding in amphibian oocytes. Furthermore, we have described the quantitative functions that relate "free" and "bound" cytoplasmic Na ion, K ion and water in full grown oocytes at 1st meiotic prophase. Cytoplasmic adsorption, cytoplasmic exclusion of diffusive solutes and the cell membrane importantly determine intracellular electrolyte levels and determine the osmotic behavior in these cells. In this proposal, we follow intracellular electrolyte kinetics and equilibria during progesterone-induced meiotic maturation, using RP-ULTM, to establish the mechanism of progesterone action, (1) in the early post-progesterone electrolyte changes; (2) in water inhibition associated with germinal vesicle breakdown; and (3) in the subsequent loss of membrane permeability. These studies directly reflect not only the mechanism of hormone action, and oogenesis, but matters as diverse as cellular membrane potentials, chemical activities, nucleocytoplasmic solute asymmetries, the mechanisms of cell volume regulation and the interpretation of exchange kinetics.